The long-term objective of this project is to ascertain the relative roles of cell lineage and cell interactions in the determination of cell fate in normal and abnormal embryonic development. The working material of this study is the embryo of the leech Helobdella triserialis. The project, which belongs to the discipline of developmental biology, is related to health in that its results are likely to provide insights into the causes of human birth defects. Its specific aims include an investigation of the morphogenetic interactions that occur between the mesodermal and ectodermal tissue layers in the development of the leech embryo. For that purpose we will make use of our cell lineage tracer techniques in combination with specific cell ablation procedures. Following ablation of the mesodermal precursor blastomere, we will examine whether the structurally disorganized ectodermal cell clumps in mesoderm-deficient regions of the embryo include any cells that, despite their lack of contact with mesoderm, have nevertheless managed to acquire one or more neurochemical characteristics of differentiated neurons. Following ablation of the ectodermal precursor blastomere, we will study those "switched" embryos in which the mesodermal stem cell bandlet has switched from the ectoderm-deprived to the unoperated side. We will examine the segmental half ganglia that have arisen while contacted by a full complement of mesoderm that contains more than half the normal number of various histologically identifiable neurons. Another specific aim is the investigation of the mechanisms underlying the phenomenon that we have designated as "transmogrification", which consists of an exchange of the respective fates of the progeney cells of two lineage-equivalent ectodermal precursor teloblasts O and P whenever the relative position of their stem cell bandlets in the germinal band happens to be inverted. By attempting to induce transmogrification at successively later developmental stages, we plan to ascertain the point at which the fate of the O- and P-teloblast-descended cells is irreversibly determined.